Diaphragms are frequently utilized in electrochemical cells to partition the cell into separate compartments. Typically, one of these compartments will house an anodic cell electrode or anode, and the other compartment will house a cathodic cell electrode or cathode.
Cell liquid containing a dissolved salt to be electrolytically disassociated is introduced into one of the compartments. An electrical potential is impressed between the anode and cathode, and this potential induces an electrical current between the electrodes. Under impetus of this electrical current, anions of the electrolyte migrate to the anode while electrolyte cations migrate to the cathode.
Where electrolyte is introduced into only one compartment, one of the ions must necessarily cross the diaphragm. Some nondisassociated components of the cell liquid normally cross the diaphragm as well.
In many cells, at least one of the ions reacts at the appropriate electrode to produce a gas that leaves the cell. Often, however, at least one of the ions reacts to form a desired chemical product soluble in the cell liquid. Often it is advantageous to concentrate this desired product for recovery.
One method of concentration is to require the ion reacting to form the soluble product to migrate, along with some of the cell liquid, through the diaphragm to the other compartment where additional reactants may be added. When this method is used, a hydraulic head is maintained in the compartment into which cell liquid is first introduced. This hydraulic head, liquid level somewhat greater than in the second compartment, promotes primarily one-way fluid movement through the diaphragm. Ions negotiating the diaphragm to react and form the desired chemical product are encouraged by one-way fluid movement fostered by the hydraulic head. Migration of the desired soluble chemical product through the diaphragm from the second compartment to the first is discouraged by the hydraulic head.
Further in some applications, the desired soluble chemical product can participate in additional and dysfunctional cell reactions if permitted to transverse the diaphragm and approach the other electrode. In these applications, division of the cell utilizing a diaphragm produces a double bonus of both concentrating the desired chemical product in a portion of the cell liquid and reducing competing cell reactions.
A typical example of such an electrochemical cell is found in the electrochemical production of chlorine. In one type of commercial chlorine process, a cell liquid or brine is formed by dissolving an alkali metal salt such as NaCl or KCl in water. The brine is introduced into the anode compartment of a suitable electrochemical cell having compartments defined by a partitioning diaphragm. When a voltage potential is impressed on the cell, anionic chlorine migrates to the anode under the impetus of the electrical potential, reacts to form chlorine gas and the gas then exits the cell by bubbling up through the cell liquid and into a collection apparatus. Alkali metal cations, normally sodium, migrate along with the cell liquid through the diaphragm to the cathode compartment and react with hydroxyl ions resulting from water decomposition at the cathode to form sodium hydroxide or caustic. The caustic is a desired chemical product soluble in the cell liquid. Hydrogen gas is also formed during this cathode reaction, and the hydrogen bubbles to the surface of the cell liquid in the cathode compartment and its captured.
The caustic generally remains dissolved in cell liquid present in the cathode compartment in an ionized state. Where the hydroxyl portions of the caustic capable of approaching the anode in significant quantity, interferring cell reactions could occur evolving oxygen by-products and reducing the cell efficiency. Maintaining of a cell liquid level in the anode compartment somewhat greater than that in the cathode compartment causes a generally uniform movement of the cell liquid through the diaphragm into the cathode compartment. This fluid movement substantially reduces the tendency for dissolved sodium hydroxide hydroxyl radicals to migrate back through the diaphragm from the cathode compartment to the anode compartment causing deleterious cell reactions.
In a typical cell, the diaphragm is exposed to chlorine on one side, caustic on the other, and is bathed with brine on both sides. As may be readily imagined, there has been a challenge in developing materials suitably resistant to these harsh chemicals and thereby acceptable for use in preparing diaphragms for use in such chlorine cells.
Traditionally, diaphragms have been manufactured from asbestos. In a typical process, asbestos fibers are slurried in an electrolyte such as caustic or brine, and the asbestos fibers are deposited as a diaphragm. Purely asbestos diaphragms have long been somewhat unsatisfactory. Such diaphragms have tended to swell and bulge after a relatively short cell on stream period. Brief upsets in cell chemical operating conditions, particularly related to pH and electrical current, accelerate degradation of the diaphragm. Even brief interruptions in the cell electrical potential contribute to relatively rapid degradation of the diaphragm.
A variety of proposals have been proffered to improve the performance and reliability of cell diaphragms.
In one commercial proposal, cell diaphragms are deposited from slurries of various Teflon.RTM. fibers, TEFLON being a product of the E. I. duPont de Nemours & Company. While TEFLON diaphragms have demonstrated a significantly improved service life over asbestos diaphragms, a major disadvantage of such TEFLON diaphragms has been their considerable cost. Wettability difficulties also have been experienced with TEFLON diaphragms. These TEFLON diaphragms have tended to be less permeable to cations and a measurably greater liquid level differential between the anode and cathode compartments has long been required to assure adequate cell liquid movement through the diaphragm and to retain caustic primarily in the cathode compartment.
It has been proposed that multiple applications of a coating of zirconyl chloride would impart improve wettability characteristics to TEFLON diaphragms, U.S. Pat. Nos. 4,170,537, 4,170,538, and 4,170,539. In this proposal, each of several coating applications individually required hydrolyzation, leaching with ammonia, and dewatering of the diaphragm to provide sequentially build up layers of ZrO.sub.2, the chemical agent attributed to enhanced wettability.
Various attempts have been made to combine asbestos and TEFLON fibers in producing diaphragms for use in chlorine cells. These attempts have met with difficulty in significant part again related to the cost of the TEFLON. Diaphragms fabricated with both TEFLON and asbestos have demonstrated improved reliability and dimensional stability over purely asbestos diaphragms.
More recently, it has been proposed that diaphragms deposited from asbestos fibers be saturated with an organic titanate, U.S. Pat. No. 4,180,449. Organic titanates saturating the diaphragm in this proposal are hydrolyzed and then pyrolyzed to yield combustion by-products and a titanium oxide residue. Diaphragms treated with pyrolyzed organo titanates have been suggested to possess additional strength and durability when compared to identical purely asbestos diaphragms to which those organo titanates have not been applied.
Difficulties have arisen in the use of organo titanates for treating diaphragms. The proposed organo titanates usually are soluble only in nonaqueous solvents. Typically, these are organic solvents often made acidic and having an elevated vapor pressure that poses a potential flammability problem when used in a manufacturing environment.
The organo titanate saturating the diaphragm is pyrolyzed to convert contained titanium to TiO.sub.2. An elevated temperature is required for this pyrolysis, often approaching 400.degree. C. This elevated pyrolysis temperature can compound difficulties with flammable solvents.
Quantities of TiO.sub.2 introduced into a diaphragm structure must be carefully monitored since TiO.sub.2 present in a diaphragm in a quantity at least in excess of 5 percent by weight potentially can reduce permeability of the diaphragm significantly during various electrochemical cell operation.